The present invention relates to devices employed for powered rotation of cylindrical or tubular members. More particularly, the present invention relates to gripping jaw assemblies, such as those found in power tongs, back-ups, wrenches and top drive casing tools, for applying controlled gripping force and rotational torque to a tubular member such as a drill pipe or casing used in subterranean well applications.
Power devices used to attach (“make-up”) and detach (“break-out”) the threaded ends of tubular members such as pipe sections and the like are commonly known as power tongs or wrenches. Such power tongs or wrenches grip the tubular element and rotate it as the end of one element is threaded into the opposing end of an adjacent element or member. A device known as a back-up is typically used in conjunction with power tongs to hold the adjacent tubular element and prevent its rotation. Power tongs and back-ups are quite similar, the major difference being the ability of tongs to rotate the tubular element. A top drive casing tool may be used to grip and rotate a section of casing.
Power tongs and wrenches generally employ a plurality of gripping assemblies, each of which includes a jaw which moves radially toward a tubular element to engage the tubular element. In the case of power tongs and wrenches, the jaw is moved radially into engagement with the tubular element and then rotated concentrically about the axis of the tubular element in order to rotate the element and therefore make-up or break-out the joint. Various mechanisms have been used in the art to actuate the jaws. Power tongs generally include devices that use interconnected gears and camming surfaces, and may include a jaw assembly which completely surrounds the tubular element and constricts concentrically in order to engage the pipe. Wrench devices generally do not completely surround the tubular element, and include independent jaw assemblies wherein the jaw assemblies may be activated by multiple, opposing hydraulic piston-cylinder assemblies. A top drive casing tool may expand jaw inserts or teeth into engagement with an inner surface of the casing.
Damage occurring to the tubular member due to deformation, scoring, slipping, etc., caused by the jaws during make-up and break-out is always a matter of concern. This scoring is of particular concern when the tubulars are manufactured from stainless steel or other costly corrosion-resistant alloys. Undesirable stress and corrosion concentrations may occur in the tubulars in the tears and gouges that are created by the tong or wrench teeth. In addition, to maintain integrity of the threaded connection, it is desirable to reduce the deformation of the pipe caused by the power tongs, wrenches and top drive casing tools near the location of the threads, thus allowing more compatible meshing of the threads and reducing frictional wear.
Increasing these concerns is the movement in the industry, particularly the well drilling industry, toward the use of new tubular members that have finer threads than those traditionally employed. Finer threads means a smaller thread pitch, making break-out harder to achieve. For these reasons, among others, it is becoming industry standard to use higher torques when making up and breaking out pipe, casing, and other tubular sections. Using the same prior art equipment and methods that have traditionally been used on older pipe may cause severe problems when used on the newer tubulars having finer threads. Therefore, with the newer, finer threaded tubulars, it is necessary to provide gripping equipment that provides enough controlled force to penetrate the pipe material, but not so much so that the pipe is irreversibly damaged.
Gouging, scoring, marring, and tearing of the pipe is typically caused when the jaws of the tong or wrench slip. Slipping may be caused by a number of undesirable conditions which cause concentration of the gripping force applied by the tong or wrench. Generally, there are two sources of slipping: the jaw clamping system and the gripping teeth. First, imperfections and flexibility in the clamping system can cause insufficient contact between gripping teeth of the tong or wrench and the pipe. When the clamping force is applied by the mechanical or hydraulic system to the jaw body, the teeth (typically formed on an insert that is retained in the jaw) engage the pipe material. However, when the torquing force is applied, thereby causing rotation of the pipe sections, a reaction force is created which pushes back on the insert. Due to the continued application of rotational force and the flexibility inherent in the hydraulic, mechanical, and other holding systems, the inserts tend to advance along and move back slightly from the pipe surface. Pin tolerances and hydraulic fluid compressibility contribute to the inherent flexibility in the holding systems. Pipe material flexibility, or elasticity, also contributes to the overall flexibility which tends to cause the inserts to creep back from the pipe. Consequently, the teeth creep back from the pipe material until there is insufficient contact between the gripping teeth and the pipe, causing the jaws to slip and mar or gouge the pipe surface. Because it is difficult to achieve a system where the jaws do not move relative to the pipe material, even in a strictly mechanical system, conventional jaws allow undesirable slipping.
A second source contributing to jaw slippage is the shortcomings inherent in the gripping teeth, which are usually set in rows on jaw inserts. The inserts are typically removable from the jaw assembly so that they may be replaced when they become worn or otherwise ineffective. Generally, assuming the clamping system is able to maintain the teeth in engagement with the pipe material, the ability of the teeth to avoid slipping is a function of the resistance that they provide. Sometimes insert resistance is viewed in terms of the resistance or penetration profile of the insert. This resistance profile represents the contact with the pipe material provided by the gripping faces of a set of insert teeth as viewed from the front of the insert in the horizontal plane in which the teeth lie. For example, evidence of pipe-scoring in tubulars held by conventional teeth inserts clearly shows a teeth profile indicating that resistance is not spread over the entire length of the tooth insert. Such scoring shows raised portions of pipe material corresponding to the spaces between the teeth where no resistance is provided. When sets of insert teeth exhibit resistance profiles with areas of no resistance, such as with conventional teeth, jaw slippage is much more likely to occur.
Therefore, it is desirable for a power tong or wrench or top drive casing tool to compensate for its inherent flexibility to prevent detrimental scoring or other damage from occurring to the tubular. It is also desirable for the gripping jaw inserts to maintain a sufficient contact area between the teeth and the pipe, and to have a more evenly distributed and fuller resistance profile.